Control of electrostatic fields



y 36, 1950 E7 M. RANSBURG ETAL 2,509,277

CONTROL OF ELECTROSTATIC FIELDS Filed April 6, 1945 4 She'ets-Sheet l I N V EN TORS. [p m/v/lZ Fmvsawae and fmewr PM; AEQ,

May 30, 1950 Filed April 6, 1945 E. M. RANSBURG ET AL CONTROL OF ELECTROSTATIC FIELDS 4 Sheets-Sheet 52 y EMEFX/QM/LlE/Q,

y 0, 1950 E. M. RANSBURG ETAL CONTROL OF ELECTROSTATIC FIELDS 4 Sheets-Sheet 5 Filed April 6, 1945 BY @v y 0, 1950 E. M. RANSBURG ETAL 2,509,277

CONTROL OF ELECTROSTATIC FIELDS Filed April 6, 1945 4 Sheets-Shet 4 Patented May 30, 1950 CONTROL OF ELECTROSTATIC FIELDS Edwin M. Ranshurg and Emery P. Miller, Indianapolis, Ind., assignors, by mesne assignments, to Ransburg Electro-Coating Corn, Indianapolis, 11141., a corporation of Indiana Application April 6, 1945, Serial No. 586,852

11 Claims.

Our invention relates to the control of electrostatic fields and more particularly to the control of electrostatic fields employed for the purpose of producing or promoting movement of finely divided materials. Such fields are used ina number of situations such, for example, as in promoting the deposition of paint'or other liquid coating material upon an article supported in or adjacent a coating spray, in precipitating dust particles from air or other gaseous medium, in removing excess paint or other coating material from a coated article, in depositing and/or orienting fibres or abrasive particles on a suitable base, etc.

In processes of the type referred to, the material to be moved acquires an electrical charge and is caused to move in the desired manner by subjecting it to the action of an electrostatic field created by maintaining a substantial potential difference between two electrodes one of which in most instances will be the article, or a conducting normal spacing, with the result that an alteration in operating conditions may result in a spark. Among factors which may cause a spark are an accumulation of material on one electrode, a reduction in the dielectric strength of the medium between the electrodes, a reduction in the spacing of electrodes, the approach of a grounded object to a charged electrode, and a surge of current in the primary of the high-voltage source. In many situations, as in an atmosphere containing dust or vapors of volatile solvents, such a spark might result in fire or an explosion.

It is the object of our invention, broadly considered, to prevent the occurrence of a spark in association with apparatus used for maintaining an electrostatic field. More specifically, it is our object to reduce or eliminate entirely the potential diflerence between two bodies whenever conditions between them approach those necessary to the occurrence of a spark from one to the other. Another object of our invention is to automatically control the potential difference across the electrodes employed to produce an electrostatic field in response to the distance between the elec-' trodes so that such potential difference will never be great enough to cause a spark from one electrode to the other. A further object of our invention is to produce a spark-preventing means 2 tion of the electrostatic field and which will not itself be adversely ailected by the operations carried on in the electrostatic field.

There are numerous ways in which our invention can be carried out. For example we may provide control means responsive to current flowing in the electrostatic field and employ such control means to modify or interrupt the operation of the high-voltage supply means whenever such current rises above a predetermined value below that which must exist before a spark can occur. As another example, we may employ means affected by any approach of the electrodes toward each other and acting to control operation of the high-voltage source in such a way as will prevent the existence across the electrodes of a potential difference great enough to cause a spark.

The accompanying drawings illustrate our invention: Fig. l is a diagrammatic view illustrating apparatus in which the potential diflerence between two spaced electrodes is controlled in response to current flowing in the electrostatic field; Fig. 2 is a diagrammatic view illustrating paintspraying apparatus in which the potential difference between two electrodes is controlled in response to the distance between the electrodes: Fig. 3 is an isometric view of a portion of the apparatus of Fig. 2; and Fig. 4 is a view similar to Fig. 3 illustrating a modified form of apparatus suitable for use in detearing.

In Fig. l we have indicated at IS an article which is to be coated with paint discharged from a manually manipulated spray gun It. Associated with the spray gun Hi there is an electrode l'l positioned so that when charged it will. impart an electrical charge to the paint particles discharged from the spray gun and will also create an electrostatic field between itself and the object l5. For the purpose of creating such an electrostatic ileld, the object i5 is grounded and the electrode I1 is connected through a conductor l8 and a rectifier l9 to one terminal of the secondary of a step-up transformer 20. The opposite terminal of such transformer-secondary is grounded, as indicated at 2|, through a resistance 22 the puzpose of which will hereinafter become apparen Except for the presenceoi' the resistance 22 in the ground lead of the transformer-secondary, the apparatus as so far described is much the same as that more fully set forth and described in the copending application of H. P. Ransburg et 9.1., Serial No. 516,778, filed January 3, 1944 which will not interfere with the normal func- 56 (now abandoned). This particular application of the use of an electrostatic field to cause movement of particles, such as the particles of a paintspray, is shown merely by way of illustration; as it is to be understood that our invention is not limited to this particular application.

In the apparatus illustrated iirFig. 1 the operation of the transformer is controlled by means responsive to the voltage across the resistance 22, such voltage in turn being responsive to the current flowing between the charged electrode l1 and the grounded article 5. The preferred means for effecting the desired control involves the use of a vacuum tube 25, shown as a triode of the gas-filled type. Associated with the triode is a power pack 26 including a potential-divider 21 providing a suitable plate voltage and grid voltage for the triode. The plate circuit of the triode 25 includes the winding of a relay 30 and that portion of the potential-divider 21 Lying between one terminal 3| thereof and an adjustable tap 32. The grid circuit of the triode includes the reslstance 22 and that portion of the voltage divider lying between the tap 32 and a second tap 33.

The relay 30 controls the holding circuit of an 'electromagnetically operated switch 35 comprising a winding 38 which, when energized, holds closed three sets of contacts 31, 38, and 39. The contacts 38 and 39 control the supply of current from a supply line 40 to the primary of the transformer 20, while the contacts 31 are connected in series with the winding 36 and the contacts of the relay 30 across the supply line. A manually operable starting switch 4| comprises a set of contacts 42 connected in parallel with the contacts 31 of the electromagneticswitch 35.

The parts of the apparatus illustrated in Fig 1 are shown in the positions they occupy when the apparatus is being operated. Y Under such condi-- tions, the contacts 42 of the manually operated switch 4| are opened while the contacts of the relay 30 and thecontacts 31, 38, and 39 of the switch 35 are all closed. Current flowing from one side of the line through the contacts of the relay 30 and the contacts 31 of the switch 35 maintains the winding 36 energized and the contacts 31,38 and 39 closed. Current flowing from the supply line through the contacts 38 and 39 energizes the transformer 20 and creates a high potential difference between the grounded object |5 and the electrode |'I. charged from the spray gun l6 acquire a. charge from the discharge electrode I1, enter the electrostatic field between that electrode and the object I5, and are attracted to the latter upon which they become deposited. Current flowing in the electrostatic field between the grounded object I5 and the charged electrode I'I passes through the resistance 22 and creates across that resistance a drop in potential directly proportional to the magnitude of the current.

The grid voltage impressed on the triode 25 will be the algebraic sum of the voltage drop across the resistance 22 and the voltage between the taps 32 and 33 of the potential-divider 21. The con-' nections are such that a negative bias will be imposed on'the triode-grid by the voltage across the taps 32 and 33and will be reduced in absolute value by the voltage across the resistance 22. The resistance 22 is such, and the taps 32 and 33 are so adjusted in relation to the characteristics of the triode 25, that no plate current will flow through the triode as long as the voltage drop across the resistance 22 remains below a predetermined maximum value lower than that created by the current which will flow through the elec- Paint particles distrostatic field immediately before a spark can occur between the object I5 and the electrode II. Accordingly, as long as the electrode ismaintained at a safe distance from the article I! the negative bias imposed on the grid of the triode 28 will remain above that necessary for current to flow in the plate circuit, the relay 30 will remain de-energized, and the operation of the trans former will continue. However, should the electrode l1 approach, the object l5 too closely, or should other conditions arise as the result of which the current in the electrostatic field is increased from its normal value toward the value which it will have just before a spark occurs, the voltage drop across the resistance 22 will increase and decrease the negative grid bias imposed on the triode 25 to a point such that the triode becomes conductive. When this occurs, current in the plate circuit energizes the plate-circuit relay 39 and thus opens the circuit through the coil 36 of the switch 95. The resultant opening of the contacts 38 and 39 de-energizes the transformer 20 and thus reduces to zero the potential difierence between the object I5 and the electrode I to remove all danger of sparking;

If desired, the switch 35 may include a fourth set of contacts 43 operated when the coil 38 is de-energized to actuate a signaldevice 45 which will apprise the operator that the potential din'erence between the electrodes l5 and H necessary to satisfactory coating no longer exists.

When the high-voltage supply apparatus is thrown out of action in the manner just described, the operator will correct the causative conditions and re-start the apparatus by closing the contacts 42 of the manually operable switch 4|. Closing of the contacts 42 completes a circuit through the contacts of the relay 39 and the actuating coil of the switch 35, and the resultant energization of the coil 36 closes the contacts 31, 38, and 39. When this occurs, the manually operable switch 4| may be released to open the contacts 42, and the coil 38 will remain energized by reason of its connection to the supply line through the contacts 31.

Immediately following closing of the contacts 39 and 39 there may be a very brief surge of current in the primary of the transformer as a result of which the secondary voltage of the transformer would for a correspondingly brief interval exceed its normal value and become great enough to cause the triode 25 to become conductive. If this should occur, the relay 39 would become energized and would open the circuit through the coil 38, thus throwing the apparatus immediately out of operation. To prevent this, we may include in the plate circuit of the triode 25 a normally closed set of contacts 41 associated with the manually operable switch 4| and arranged to be opened when that switch is operated to close the contacts 42. With such an arrangement any transient current surge causing excessive voltage acros the terminals of the secondary of the transformer 20 would not result in energization of the relay 30 as the plate circuit, in which the winding of such relay is connected would be potential diflerence between the object II and electrode I1, and requires a manual operation for re-starting. It is also to be noted that the control of the high voltage apparatus is exercised in response to current variations in the electrostatic field. As a result, the high voltage apparatus will be rendered inoperative whether the rise of current to a dangerous value results from too short a distance between the object I5 and the electrode II or from some other cause. It is not essential in the practice of our invention that the voltage between the object I5 or its equivalent and the electrode ll be reduced completely to zero upon the occurrence of conditions conducive to sparking, nor is it essential that the control of the high voltage apparatus be exercised in response to current.

In situations where it is desired to guard against sparking resulting only from a too close approach to an electrode of an object having a different potential, the high voltage apparatus may be controlled directly in response to the distance between the electrode and such an article. This method of control is illustrated in Figs. 2, 3, and 4 where we have shown it as being used in conjunction with electrostatic coating apparatus of a type more fully describedin United States Letters Patent No. 2,247,963, issued July 1, 1941, on the application of H. P. Ransburg and Harry J. Green. In the spray-coating apparatus illustrated in Figs. 2 and 3, a succession of articles 58 suspended from an overhead conveyor 5| are passed through an electrostatic field created by an electrode 52 while paint is sprayed into that field from spray guns 53. The electrode 52 is in the form of an open-ended cage suspended by insulators 54 and comprising an open frame 55 which supports a plurality of relatively flne wires iii.

In apparatus of the kind just referred to, any swinging of the articles 58 as they pass through the cage would reduce the distance between such articles and the cage; and if the swinging were sufficiently violent, a spark might result. Similarly, if an object should accidently become detached from its overhead support and drop downward in the cage, a spark might result. To guard against these possibilities we provide means actuated upon movement of any article 58 toward the electrode 52 for reducing the potential difference between the object and the electrode. In the specific embodiment shown, this reduction is not an immediate reduction to zero as in the case of the apparatus shown in Fig. 1 but instead is a progressive reduction dependent in amount upon the approach of the article to the electrode.

We find it convenient to employ an arrangement of light beams and photoelectric cells in situations where the high-voltage source is to be controlled directly in response to variations in the distance between the electrode and the' article. In Fig. 3 we have illustrated a series of horizontally spaced light-sources 88, shown as four in number, arranged to project four parallel, horizontally spaced light-beams GI, 82, 63, and 88 longitudinally of the cage 52 and between one side thereof and the articles passing therethrough. At the remote end of the cage such light beams are intercepted by a system of mirrors 85 and reflected longitudinally of the cage between the article 58 and the cage-bottom. A second set of mirrors 85a. again reflects the light beams longitudinally of the cage beneath the articles 58 passing therethrough, and a third set .8 the cage a fourth time and to be received respectively in photoelectric cells 88. 81, 88, and 88. It is to he noted from Fig. 3 that the order of the light beams. proceeding from the one nearest the object 88 outwardly toward the electrode 52, is unchanged by the various reflections of. the beams. Accordingly, irrespective of the direction in which the article 58 may approach the electrode it will always interrupt the light beams in the same order.

The wiring diagram constituting Fig. 2 illustrates the circuits controlled by the light beams 8|, 82, 88, and 58. As there shown, the photoelectric cells 86, 81, 88, and 88 are respectively connected-through amplifiers with the windings of relays II, II, I8, and I4 to maintain the re lays closed when light-beams impinge on the cells. and each of the relays is of the type whose contacts are closed when the relay-winding is energized. "Associated respectively with the relays II, I2, I8. and II are four electromagnetic switches I8, 11, I8, and I8. The switch I8 embodies two sets of contacts 88 and 8|, which are closed when the switch is energized and open when the switch is de-energized. and two additional sets of contacts 82 and 83, which are open when the switch is energized and closed when it is de-energized. The two switches 11 and I8 are identical, each comprising three sets of contacts 84. 85, and 88, which are closed when the switch is energized and open when it is de-energized, and two other sets of contacts 81 and 88, which are open when the switch is energized and closed when it is tie-energized. The fourth switch, I9, includes four sets of contacts 88, 9|, 92, and 88, all closed when the switch is energized and open when itisde-energized.

The apparatus illustrated in Fig. 2 comprises the same step-up transformer 28 as does the apparatus illustrated in Fig. 1. The electrode 52 is connected through the rectifier III with one terminal of the transformer secondary, while the opposite terminal of the transformer secondary and the articles 58 are grounded. As previously indicated, the control of the potential diiference between the articles 58 and the electrode 52 is a progressive control which we preferably effect through the medium of an auto transformer I88 connected across the supply wires I8I and I82 and provided with three equally spaced intermediate taps I83, I84, and I85. The terminals of the transformer 28 may be connected to the supply wires I8I and I8: or taps I83, I88, and I over any of a plurality of paths. Thus, the primary terminal I8I may be connected to the supply line I8I through the switch contacts 82 of switch I8, 81 of switches 11 and I8, and 82 of switch I8; through contacts 85 of switch I8, 81 of switch TI, and 82 of switch 18; through contacts 85 of switch 11, 82 of switch 18; or through contacts 8| of switch I8. The opposite terminal I88 of the transformer primary may be connected through contacts 93 of switch I8 and contacts 88 of switch I8 to tap I85 of the auto transformer I88; through contacts 88 of switch I8 and contacts 88 of switch 11 to the middle tap I84 of the auto transformer I88; through contacts 88 of switch II and contacts 83 of switchfli to tap I83 of the auto-transformer; or through contacts 88 of switch I8 to the supply wire I82.

The winding of each of the electromagnetic switches I8, 11, I8, and I8 is connected across the supp y wires I8I and I82 through the contacts of the associated relay II, I2, I3, and ll.

of mirrors 8512 causes the light beams'to traverse 1 In addition, the circuit of the winding 01 the 52 parallel to and without interrupting any of the light beams BI, 82, 88, and 88. As all the photoelectric cells 88, 81, 88, and 88 are receiving light, the relays H, l2, l8, and 14 are all energized, as are also the windings of the electromagnetic switches 18, 11, I8, and I8. In this condition, the transformer receives current over a circuit comprising line wire llll, contacts 8| of switch I8, primary terminals I81 and H18, contacts 88 0! switch 16 and line I82.

Should any of the articles 58 passing through the cage electrode 52 swing toward the sides of the cage or become dissociated from its support and drop toward the bottom of the cage, it would first interrupt the light beam 8|. Upon interruption of the light beam H, the relay 1| would be de-energized, thus opening the circuit through the winding of switch I8 and causing separation of each set of contacts 80 and 8| and engagement of each set of contacts 82 and 88. If movement of the article 50 is insuflicient to interrupt the next light beam 62, the transformer-primary will then be connected across three-fourths of the winding of the auto transformer ill, or between the line wire I 0! and tap I08, through contacts 82. and 83 of switch 18 and contacts- 85 and 88 of switch 11. Because the transformer now receives its current through the auto transformer, the primary voltage of the transformer is reduced to effect a corresponding reduction in the potential difference between the article 58 and the electrode 52. Should the article 58, after interruptingthe light beam 6|, move back to its normal position to permit such light beam again to impinge on the photoelectric cell 88, the relay H will be energized to cause energization of the winding of the switch 16, thus restoring the contacts of that switch to the condition illustrated in Fig. 2 where the voltage impressed on the transformer primary is at its original value.

If, instead of returning to its normal position outside the path of the light beam 8| after interrupting such light beam, movement of the article 58 toward the electrode continues, the second light beam 62 will be interrupted to cause de-energization of the relay 12. when this occurs, the winding of the electromagnetic switch 11' will be de-energized, the three sets of contacts 88, 85, and 86 associated with that switch will be separated, while the two sets of contacts 81 and 88 will be closed. when this occurs, the transformer primary becomes connected across onevhalf the winding of the auto transformer, or between the line wire llll and tap I, through the ontacts 82 of switch 18, contacts 81 of switch ll, contacts 85 of switch 18, contacts 88 of switch 18, and contacts 88 of switch 11. As a result, both the primary and secondary voltages of the transformer 20 will be reduced to one-half their normal value.

If movement of the article 58 continues to a point where the beam 63 is interrupted, the photoelectric cell will act to de-energ'ize the relay 18 thus opening the circuit through the winding of the electromagnetic switch 18. As a result of the de-energization of such winding, the sets of switch contacts 88, 88. and" of that switch will all be separated while the sets of .contacts 81 and 88 will be closed. When this occurs, the transformer-primary will become connected between line wire Ill and tap I05, through switch contacts 82 of switch I8, contacts 81 of switch 'l'l, contacts'll' of switch 18, contacts 82 of switch 19, switch contacts 88 of switch 18, contacts 88 ofswitch II, to tap I05. In this condition, both primary and secondary transformer voltages are reduced to one-fourth their normal value.

If the object 58 should movev far enough to interrupt the outside light beam 88, the relay l8 will be de-energized to open the circuit through the winding of switch 18, thus separating the-contacts of each set 88, 8|, 82, and 88 associated with such switch. When this occurs, the suppw of current to the transformer-primary will be completely interrupted, and the potential difference between the article 50 and electrode 52 will be reduced to zero. After the light beam 88 is once broken, removal of the article ID from its path will not result in energization of the transformer 20; for the circuit of the winding of each of the switches I6, 11, and 18 willremain open because of the separation of contacts associated with the electromagnetic switch of next higher number while the circuit of the winding of the switch 19 will be open because of the separation of the set of contacts 88. To restore the apparatus to operative condition it will therefore be necessary to do so manually rather than automatically. For this purpose, we have provided the normally open switch H8 which is connected across the set of contacts 98 of the switch 18 and which will, when temporarily closed, complete a circuit through the winding of switch 18 if the light beam 68 is uninterrupted and the relay l4 energized. Energization of the switch 18 will cause engagement of the sets of contacts 98, 8|, 92, and 93, and the engagement of a the set 9| will permit energization of the switch 18 if the light beam 83 is uninterrupted. In such manner, the energization of each electromagnetic switch permits the energization of the next lower numbered switch if the light beam controlling the same is uninterrupted; and as a result, if all the light beams are uninterrupted when the switch H8 is closed, all the relays ll, l2, l8, and I4 and all the electromagnetic switches will become energized to restore the apparatus to the condition illustrated in Fig. 2 in which the full normal potential difference exists between the article 50 and the electrode 52.

It is to be noted that if the dimensions of the article 58 are insuflicient to permit it to interrupt more than one light beam at a time, the operation of the apparatus as above described will not be interfered with. For example, if in moving to interrupt the light beam 62 the article 58 clears the light beam 8| the potential difference between the article 88 and the electrode 52 will be the same as if both light-beams iii and 62 were interrupted; because with the light beam 62 interrupted the switch I1 will be de-energized, its contacts 88 will be separated, and it will be impossible to energize the winding of switch I8.

It is evident that if movement of the article 58 toward the electrode 52 stops before the lightbeam 88 is interrupted and if the article then returns to normal position, the potential difference between the article and electrode will be progressively increased as the article clears successive light-beams. Only when the outermost 9 light beam BI is interrupted will the specific apparatus shown in the drawing require manual resetting; and, if desired, the necessity for manual resetting can be eliminated altogether by eliminating the switch-contacts 90 from thecircult of the winding of the switch ll.

Our invention is applicable in a wide variety of situations and irrespective of the particular use to which the electrostatic field is put. The apparatus of Fig. 1 is usable without change, other than appropriate adjustment of its electrical characteristics, irrespective of the character of the electrodes between which the electrostatic field is to be created. In the apparatus-oi Pig. 2, however, the light-sources 6B, the mirror system- (it any), and the photo-electric cells SI, 02, i3, and 64 will of necessity be arranged with regard to the shape and relative disposition of the electrodes. For example, in detearing, or removing excess coating material from articles coated in any desired manner, it is customary to create an electrostatic field between the articles and an open-work grid over which they are successively passed. In such apparatus, one form or which is illustrated in Fig. 4, we may arrange the lightsources 60 and mirrors 65' so that the light beams will repeatedly traverse the electrostatic field in position to be interrupted by an object 50 which falls from the overhead conveyor 5! or which, for any other reason, approaches dangerously close to the open-work grid or electrode 52'.

It may be noted that the structures illustrated in the drawings will operate to prevent sparks whether or not such sparks tend to occur between the two bodies which are considered as electrodes in that they are connected to opposite terminals of a high-potential source. Thus, interruption of a light beam in the apparatus of Fig. 2 or Fig. 4 will reduce the potential of the electrode 52 or 52 whether or not the body causing such interruption is one of the articles 50 being treated. In the apparatus oi. Fig. 1, any grounded body approaching the electrode IT or any other part Of the apparatus at a high potential will tend to increase the current flowing in the electrostatic field and through the resistance 22, and hence will cause the triode 25 to become conductive and reduce the potential of the electrode.

Our invention not only promotes safety but also economy. In prior apparatus of the general type with which our invention is concerned, it has heretofore been a frequent practice to space the electrodes at a greater distance apart than would otherwise be necessary, in order to reduce the danger oi sparking if the distance between them should be accidentally reduced. Such augmented spacing of the electrodes for safety purposes has made it necessary to increase the potential difference between the electrodes if the strength of the electrostatic field between them is not to be reduced. Through the use of our invention, it becomes possible to reduce greatly or to eliminate altogether any increment of electrode spacing which would otherwise be necessary as a safety factor; and therefore an electrostatic field of. desired strength may be produced by the use of lower voltages.

One advantage of apparatus, such as that of Fig. 1. in which the potential difference between the electrodes is controlled in response to field current, arises from the fact that the slope of the current curve plotted against any condition affecting the tendency of a spark to occur increases very rapidly as conditions approach those ratus including control mechanism responsive to field current may be designed to operate under conditions very close to those under which a spark will occur.

It is to be understood, of course, that our invention is not limited in respect to the manner in which the oppositely charged bodies are connected to the high-voltage source. For example. in Fi 1 the article ll could be connected to the conductor It and the electrode ll grounded, and the control apparatus would operate with equal ellectivcness to reduce the potential diilerence as conditions between the article and electrode approached those necessary to the occurrence of a spark. It is likewise to be understood that our invention is not limited to processes or apparatus in which an article to be treated forms one or the electrodes between which the electrostatic field is created or in which the particles moved by the actionoi the electrostatic field are liquid particles.

We claim as our invention:

1. In a method for producing an electrostatic field in a gaseous medium, the steps of maintaining a high potential dlflerence between bodies spaced in such medium, directing a light-beam along a path lying between and spaced from said bodies at a predetermined distance from one of intercepts said beam.

3. In an apparatus for producing an electrostatic field between two objects, a means of controlling the magnitude of said field, comprising a resistance element in the circuit containing the source of said field, an electron tube arranged to be biased proportionately to the voltage drop across said resistance and means operat-ively associated with said tube to control the supply to said field source.

4. Apparatus for producing an electrostatic field between two objects comprising a high voltage source having an input circuit adaptedto be electrically connected to an electrical supply and an output circuit electrically connected to the two objects, a resistance means in said output circuit, and an electronic switch biased proportionately to the voltage drop across said resistance means to deenergize said input circuit when said voltage drop exceeds a predetermined value.

5. A control system comprising, in combination, an electrical load. means for energizing said electrical load with a unidirectional current of high potential, a power supply for said energizing means, a resistance means in circuit with. said energizing means and said electrical load across which a potential difierence of predetermined value is created just prior to an overload condition in its circuit, a grid controlled gas tube arranged to be biased proportionately to the potential difference across said resistance means and being activated when said potential difference reaches said predetermined value, and means responsive to the activation of said tube 11 for disconnecting said power supply and said energizing means.

6. Apparatus for controlling the strength of an electrostatic field employed in promoting the movement of finely divided coating material between an article and an electrode spaced from the article, a power supply, a high voltage source having an input circuit electrically connected to said supply and an output circuit having its terminals associated respectively with the article and the electrode, a resistance means in said output circuit, and an electron switch biased proportionately to the potential difference across said resistance means and being responsive to a predetermined value of such potential difference across said resistance means to disconnect said suppl from said high voltage source.

7. In combination with electrostatic coatin apparatus including an article to be coated which serves as a collecting electrode, a discharge electrode disposed in spaced relation to said collecting electrode, coating material issuing means for spraying coating material into the space between said discharge electrode and the article for electrostatic deposition on the article, a, transformer having a primary and secondary winding, a rectifier for receiving alternating current from said secondary, means for connecting said rectifier to said discharge electrode and the article, a supply of alternating current for energizing said primary, a resistance means in said connecting means, a thyratron arranged to be biased proportionately to the voltage drop across said resistance means and being activated" in response to a predetermined value of such potential difference across said resistance means, a switch in said primary circuit responsive to the activation of said thyratron for interrupting the current flow to said primary.

8. In apparatus for coating an article in an electrostatic field, a discharge electrode spaced from the article to be coated, a source of unidirectional current of a high potential electrically connected to said electrode and the article being coated for creating an electrostatic field therebetween, an alternating current power supply connected to said source, a resistance means connected to the circuit includin said source and said electrode, a grid controlled gas tube biased proportionately to the voltage drop across said resistance means and being responsive to a predetermined potential difference across said resistance means for activating said tube, means associated with the grid of said tube for varying the value of the potential difference to which said tube respond for activation, and means for dis- 12 connecting said supply and said source in response to the activation of said tube.

9. In an apparatus for producing an electrostatic field between an electrode and an article spaced therefrom, a light source disposed to cast a beam of light across said field along a path lying between and spaced from the electrode and the article and at a predetermined distance from the electrode, a photosensitive device positioned to receive the light beam, and control means actuated by said device for controlling the strength of the field when said light beam is intercepted.

10. In an apparatus for producing an electrostatic field between an electrode and an article spaced therefrom, said electrode and article being movable relative to each other, the combination comprising a series of light sources disposed to cast beams of light across said field and at dif- Ierent predetermined distances from the electrode, each of said beams following a path lying between and spaced from the electrode and article, a photosensitive device positioned to receive each light beam, and control means actuated by said devices for progressively reducing the strength of the field when said light beams are successively interrupted.

11. In apparatus for electrostatically removing excess material from a coated article, spaced from an electrode by an electrostatic field existing between the article and electrode. a light source disposed to cast a beam of light across said field along a path lying between and spaced from the article and electrode, and at a predetermined distance from the electrode, a photosensitive device positioned to receive the light beam, and control means actuated by said device for controlling the strength of the field when said light beam is intercepted.

EDWIN M. RANSBURG. EMERY P. MILLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

